Polycarbonates are a well known class of high impact resistant thermoplastic resins characterized by optical clarity, high ductility as well as other advantageous properties. They are frequently employed as lenses and windows as a result of their transparency. Bisphenol A polycarbonate (BPA) is the predominant commercially available resin of this type. It is derived from 2,2-bis(4-hydroxyphenyl)propane, and ordinarily has a glass transition temperature of about 150.degree. C.
It is of increasing interest to prepare polycarbonates which, while retaining the ductility of bisphenol A polycarbonates, have higher glass transition temperatures and are therefore more resistant to softening when heated. Moreover, there is need for polycarbonates which possess flame retardant properties since they are, for instance, conventionally used in the automotive and aircraft industries. Several flame retardant agents have been utilized in an attempt to produce flame retardant polycarbonates. For example, alkali metal salts of strong sulfonic acids are commonly used. However, when incorporated into the polycarbonate, the resulting polymer is hydrolytically sensitive. Further, when using these salts, it is also necessary to employ drip inhibitors or gas phase flame retardant agents. This is not ideal since drip inhibitors destroy the clarity of the polymer and gas phase retardants are often halogenated which creates problems with corrosion and toxicity. As an alternative to the above, phosphorus containing compounds such as triphenylphosphate have been used. When blended with a base polycarbonate, some flame retardant properties are observed. However, the resulting polymer blends are not desirable since they possess low glass transition temperatures (Tg) and low impact resistance when compared to the base resin.
The present invention is based on the discovery of bis[2,5-(diphenylphosphine oxide)]-1,4-hydroquinone and homologs thereof, and their incorporation into polycarbonates. The resulting bisphosphine oxide substituted polycarbonates are expected to exhibit improved flame retardancy since they possess phosphine oxide groups which increase polycarbonate limiting oxygen index values. Additionally, they are expected to retain high Tg values and impact resistances when compared to the base resin devoid of the bisphosphine oxide comonomer.